Light emitting diodes (LED's), lasers, and the like (collectively known as light emitting devices) are used widely in many applications today such as communications systems, medical systems, and display systems. These light emitting devices are commonly fabricated with epitaxial materials formed on a substrate, the epitaxial materials having a p-n junction, or active region, formed therein and include at least one Bragg reflector. A Bragg reflector is a fundamental building block of many light emitting devices. Typically, in LED's, a Bragg reflector is fabricated between an active region and a substrate, and in lasers, a Bragg reflector is fabricated on either side of an active region. In the case of an LED, light emitted from an active region toward the substrate is reflected by a Bragg reflector back toward the surface where it combines with the light going toward the surface, thus increasing the light output of the LED. Bragg reflectors are typically composed of alternating layers of material having different refractive indices.
LED's and lasers are typically fabricated by growing semiconductor layers on a substrate material. The portions of the semiconductor layers that reside within the optical and electrical path of the device should be relatively defect free.
An active region is typically formed from epitaxial growth material, which can be grown over a mask to form reduced defect growth material. The semiconductor material is typically fabricated by growing an epitaxial layer of a chosen material over an oxide layer upon a substrate material. The substrate material may be, and frequently is, of a different composition than the material used to grow the epitaxial layer.
The epitaxial layer is typically a thin single crystalline film that is deposited upon a crystalline substrate. The epitaxial layer is typically deposited so that the crystal lattice structure of the epitaxial layer closely matches the crystal lattice structure of the substrate. When there is a significant mismatch between the lattice structure of the substrate and the epitaxial layer, a large number of defects, or dislocations, can result. Dislocations manifest in the form of imperfections in the crystal structure and can result in high optical loss, low optical efficiency, non uniform quantum wells in the active region, or the reduction of the electrical quality of the material, thus preventing the material from being used to fabricate certain devices, such as lasers and transistor structures. A largely dislocation-free material is desired for these highly critical devices.
Generating blue light is an important application of many of the LED's and lasers discussed herein. Blue light devices are generally based upon gallium nitride (GaN) materials, however, it is difficult to grow relatively defect free gallium nitride material.
Dislocation density can be reduced by growing an epitaxial lateral growth layer of gallium nitride material over a mask layer. When the epitaxial layer is then grown over the mask, the epitaxial layer grows laterally, resulting in a reduced dislocation density being present in the portion of the epitaxial layer that resides over the mask. Because the dislocations tend to propagate vertically, the vertically grown material present in the unmasked region of a wafer will be of higher dislocation density as the defects will continue to propagate throughout the layer.
Thus, an unaddressed need exists in the industry for a simplified method for growing a relatively defect free semiconductor material for a light emitting device that includes a reflector, or mirror, therein.